Foster et al., in U.S. Pat. No. 3,919,348, taught epoxy-styrene-anhydride resins useful for vacuum pressure impregnation of mica insulated motor and generator coils. These resins relied upon azo compounds and peroxide catalysts to cure the resin. These resins provided a low viscosity impregnant, with excellent electrical and storage properties, but required either a high temperature final cure at about 170.degree. C., or a long, 16 hour final cure at about 150.degree. C., in order to provide power factor values of about 10% to 13% at 150.degree. C. The high temperature cure could cause mica delamination from copper conductors due to their difference in thermal expansion characteristics. A long, lower temperature cure, boosts energy requirements and slows the production shop schedule.
Markovitz, in U.S. Pat. No. 3,812,214, taught control of the curing rate of viscous epoxy resin systems, by using up to about 20 wt.% of a combination of a phenolic resin accelerator with a metal acetylacetonate acting as a catalytic hardener. Markovitz eliminated anhydride curing agents, and substitued the phenolic resin, and up to 5 wt.% of metal acetylacetonates. The useful acetylacetonates could contain essentially any metal anion. Naps, in U.S. Pat. No. 2,876,208, and Starck, in U.S. Pat. No. 2,801,228, used copper acetylacetone as a stabilizer for phenolic resins, and nickel acetylacetone as a hardener for epoxy resins, respectively.
Miyashiro, in U.S. Pat. No. 3,624,032, taught epoxy-anhydride, particulate transfer molding compositions for encapsulating electronic parts. These compositions contained selected metal acetylacetonates to promote reactivity and bring about a high quality cure at about 150.degree. C. In the Mlyashiro system, the metal anion was selected from zinc, aluminum, iron, copper, cobalt, nickel, magnesium, zirconium and tin.
What is needed is a low viscosity, styrene containing impregnant, containing a compatible accelerator not reactive with the styrene, or peroxide or azo catalysts, which will provide good pot life, yet quick, low temperature cures, and resulting power factors of below about 15% at 150.degree. C.